1. Field of the Invention
This invention relates generally to well logging and more particularly to use of acoustic waves to obtain information about a borehole and the surrounding subsurface structure.
2. Description of the Prior Art
In the exploration for and the production of oil and gas it is desirable to discover as much as possible about the subsurface structure of the earth in the area of interest. Once a well has been drilled, it is typical to attempt to gather data about the subsurface structure from the borehole itself. One parameter of the subsurface structure which influences the quantity of oil and gas that can be extracted from a reservoir, and the speed at which the oil and gas can be extracted, is the permeability of the formation.
It is known that sonic waves traveling within a wellbore, sometimes referred to as Stoneley waves and sometimes as tube waves, will be more strongly attenuated while traveling through a borehole surrounded by a more permeable formation than through a borehole surrounded by a less permeable formation. See, for example, "Synthetic Full Waveform Acoustic Logs in Cased Boreholes", Geophysics vol. 49, No. 7, July 1984 pp. 1051-1059; and "Application of Full Waveform Acoustic Logging Data to the Estimation of Reservoir Permeability" presented at the 54th Annual SEG Convention, 1984.
In the past, wave attenuation has been measured by detonating a small dynamite charge in a shallow hole near the well head and recording the resulting signal generated in the wellbore with detectors such as hydrophones.
Stoneley waves have also been measured by use of ceramic transmitters and receivers, wherein eight receivers are arranged in an array with 15 cm. between each receiver, and 3.048 m. (10 ft.) from the transmitter to the first receiver. "Vp/Vs in Unconsolidated Oil Sands: Shear From Stoneley", Geophysics, Vol. 52 No. 4, April 1987, pp. 502-513.
Methods of measuring the tube (or Stoneley) wave known to the prior art for evaluating the permeability of a formation have consisted of generating a sonic signal and detecting the magnitude of the wave as it travels past detectors.
Recently, there has been disclosed in U.S. Pat. No. 4,671,379, Kennedy et al, "Method and Apparatus for Generating Seismic Waves", issued June 9, 1987, a source for generating a seismic signal in a borehole. The source includes first and second end means, or operating heads, which are spaced a selected distance apart. The space between the two end means is filled with borehole fluid. Oscillatory or pulsed means may be utilized to excite the fluid between the two end means. By correlating the excitation frequency with the distance between the two end means, a standing wave may be generated. The patent discloses detection of the signal at the surface in order to evaluate the nature of the subsurface structure through which the seismic signal passes between the location of the source in the borehole and the location of the detectors on the surface. It has not, however, been previously suggested that a source for generating standing waves could be utilized for logging permeability variations of a subsurface formation.